1. Field of the Invention
This invention relates to a process for the preparation of doped anionic clays, and doped anionic clays prepared by that process.
2. Prior Art
Anionic clays have a crystal structure that comprises positively charged layers built up of specific combinations of metal hydroxides between which there are anions and water molecules. Hydrotalcite is an example of a naturally occurring anionic clay, in which carbonate is the predominant anion present. Meixnerite is an anionic clay wherein hydroxyl is the predominant anion present.
In hydrotalcite-like anionic clays the brucite-like main layers are built up of octahedra alternating with interlayers in which water molecules and anions, more particularly carbonate ions, are distributed. The interlayers may contain anions such as NO3−, OH, Cl−, Br−, I−, SO42−, SiO32−, CrO42−, BO32−, MnO4−, HGaO32−, HVO42−, ClO4−, BO32−, pillaring anions such as V10O28−6 and Mo7O246−, monocarboxylates such as acetate, dicarboxylates such as oxalate, alkyl sulphonates such as laurylsulphonate.
It should be noted that a variety of terms are used to describe the material that is referred to in this specification as an anionic clay. Hydrotalcite-like and layered double hydroxide is interchangeably used by those skilled in the art. In this specification we refer to these materials as anionic clays, comprising within that term hydrotalcite-like and layered double hydroxide materials.
The preparation of anionic clays has been described in many prior art publications. Two major reviews of anionic clay chemistry were published in which the synthesis methods available for anionic clay synthesis have been summarized: F. Cavani et al “Hydrotalcite-type anionic clays: Preparation, Properties and Applications,” Catalysis Today”, 11 (1991) Elsevier Science Publishers B. V. Amsterdam; and J P Besse and others “Anionic clays: trends in pillary chemistry, its synthesis and microporous solids”(1992), 2, 108, editors: M. I. Occelli, H. E. Robson, Van Nostrand Reinhold, N.Y.
In these reviews the authors state that a characteristic of Mg—Al anionic clays is that mild calcination at 500° C. results in the formation of a disordered MgO-like product. Said disordered MgO-like product is distinguishable from spinel (which results upon severe calcination) and from anionic clays. In this specification we refer to said disordered MgO-like materials as Mg—Al solid solutions. Furthermore, these Mg—Al solid solutions contain a well-known memory effect whereby the exposure to water of such calcined materials results in the reformation of the anionic clay structure.
Two types of anionic clay preparation are described in these reviews. The most conventional method is co-precipitation (in Besse this method is called the salt-base method) of a soluble divalent metal salt and a soluble trivalent metal salt, optionally followed by hydrothermal treatment or aging to increase the crystallite size. The second method is the salt-oxide method in which a divalent metal oxide is reacted at atmospheric pressure with a soluble trivalent metal salt, followed by aging under atmospheric pressure. This method has only been described for the use of ZnO and CuO in combination with soluble trivalent metal salts.
For work on anionic clays, reference is further made to the following articles:    Chemistry Letters (Japan), 843 (1973)    Clays and Clay Minerals, 23, 369 (1975)    Clays and Clay Minerals, 28, 50 (1980)    Clays and Clay Minerals, 34, 507 (1996)    Materials Chemistry and Physics, 14, 569 (1986).In addition there is an extensive amount of patent literature on the use of anionic clays and processes for their preparation.
Several patent applications relating to the production of anionic clays from inexpensive raw materials have been published. These materials include magnesium oxide and aluminum trihydrate.
WO 99/441198 relates to the production of anionic clay from two types of aluminum compounds and a magnesium source. One of the aluminum sources is aluminum trihydrate or a thermally treated form thereof.
WO 99/41196 discloses the preparation of anionic clays with acetate as the charge balancing anion from magnesium acetate, another magnesium source and aluminum trihydrate.
In WO 99/41195 a continuous process is described for the production of a Mg—Al anionic clay from a Mg source and aluminum trihydrate.
WO 99/41197 discloses the production of an anionic clay-containing composition comprising a Mg—Al anionic clay and unreacted aluminum trihydrate or a thermally treated form thereof.
Several patents describe the synthesis of hydrotalcites, i.e. anionic clays, out of magnesium oxide and a transition alumina in a batch-wise manner and under non-hydrothermal conditions: U.S. Pat. Nos. 5,728,364, 5,728,365, 5,728,366, 5,730,951, 5,776,424, 5,578,286. Comparative Examples 1–3 presented in these patents indicate that upon using aluminum trihydrate as aluminum source anionic clays are not formed.
There are many applications of anionic clays. These include but are not restricted to: catalysts, adsorbents, drilling muds, catalyst supports and carriers, extenders and applications in the medical field. In particular Van Broekhoven (U.S. Pat. Nos. 4,956,581 and 4,952,382) has described their use in SOx abatement chemistry.
For several applications the presence of additives, both metals and non-metals, within the anionic clay is desirable. These additives are used to alter or enhance certain properties of the anionic clay. For instance, Ce and V are added to the anionic clay to obtain material suitable for SOx removal in FCC. In general, these additives are deposited on the anionic clay by impregnation. With impregnation, however, it is often difficult to obtain a homogeneous dispersion of the additive within the anionic clay or it is difficult to deposit enough additive on the anionic clay to obtain the desired properties.
Some patent publications indicate that the additives may be added to the reaction mixture during preparation of the anionic clay. However, when additives are added to the reaction mixture, their presence may interfere with the anionic clay formation. For instance, when anionic clays are made by co-precipitation it is possible that the pH required to precipitate for example the divalent and the trivalent metal source may not be optimum for precipitation of the additive. In extreme situations the additive may be precipitated in advance of the divalent and trivalent metal sources or may not be sufficiently precipitated and remain in solution. Hence, with co-precipitation it is also difficult to control the amount and the dispersion of the additive in the anionic clay.